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Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI = molecular Al/[Na + K]) of the dykes is 1.20 ( n = 29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na-Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb-Sr isochron age of 171 ± 4.4 Ma, with variable initial Sr/Sr ratio (0.7075 ± 0.0032), and have an average ε of − 12.0. Quartz phenocrysts have δO values of 8.4-9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δO values of quartz and aegirine (average Δ = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative ε (< − 10) and magma δO values averaging 8.6‰ (assuming Δ = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δO values and much less negative ε (average − 3.1, n = 3). The country rock has similar δO values to the granite dykes (average 8.0‰, n = 108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δO contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization. [ABSTRACT FROM AUTHOR]

Fluorine and chlorine affect phase stabilities in magmatic rocks. We present phase equilibrium experiments investigating a peralkaline and iron-rich phonolitic composition with variable F and Cl contents. The starting composition represents a dyke rock, which is a possible parental melt to the peralkaline Ilímaussaq plutonic complex (South Greenland). Experiments were performed at 100 MPa, 1,000-650 °C and low oxygen fugacity adjusted with graphite-lined gold capsules in an internally heated argon pressure vessel and rapid quench cold seal pressure vessels. To cover this large T interval, we applied a two-step fractional crystallization strategy where glasses representing residual melt compositions at 800 °C were synthesized as starting material for consecutive experiments at lower T. In these experiments, oxidized starting glasses allocate oxygen by reduction of ferric iron and up to 1.2 wt% dissolved OH form through reaction with hydrogen provided by the pressure medium (HO) in initially dry experiments. For OH determination, hydrated super-liquidus experiments in Au capsules were performed to calibrate the extinction coefficient for the fundamental OH stretching vibration using infrared spectroscopy ( ε = 48 ± 3 L mol cm). Observed mineral phases in our experiments are titanomagnetite, fayalitic olivine, clinopyroxene, aenigmatite (NaFeTiSiO), alkali feldspar and nepheline (±native iron) coexisting with residual melt. Above 1.5 wt% F concentrations, fluorite (CaF) and hiortdahlite (CaZrSiOF) are stable in favor of Ca-rich clinopyroxene. Sodalite (NaAlSiOCl) and eudialyte (NaCaFeZrSiO(OH)Cl) form at Cl concentrations of 0.2-0.5 wt% (depending on T) and ZrO concentrations >0.7 wt% are additionally needed to stabilize eudialyte and hiortdahlite. Therefore, F and Cl may become compatible in such systems and have the potential to influence F/Cl melt ratios in evolving magmas. [ABSTRACT FROM AUTHOR]

Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na2O + K2O) < Al2O3] and peralkaline [(Na2O + K2O) > Al2O3] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe–Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (± ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite + aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na–K–Ca–(Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na–K–Ca–(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (µNa2O, µCaO, µK2O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution. High-Ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase. In contrast, agpaites showing Ca-depletion originate by extensive fractionation of plagioclase from basaltic parental melts. In some peralkaline granites evolutionary trends are observed that culminate in agpaite-like HFSE-mineral associations in the most evolved rocks. [ABSTRACT FROM AUTHOR]